1. Field of the Invention
The present invention relates to a rolled copper foil, more particularly, to a rolled copper foil to be used for a flexible printed circuit (FPC) or the like.
2. Description of the Related Art
The FPC is provided with a high degree of freedom in mounting morphology for electronic device or the like, since the FPC has a reduced thickness and superior flexibility. Therefore, the FPC has been used for a bending part of a folding type portable phone, a movable part of digital camera, printer head or the like, an electric wiring of a movable part of disc-related equipment such as Hard Disk Drive (HDD), Digital Versatile Disc (DVD), Compact Disk (CD).
Japanese Patent Laid-Open No. 2002-167632 (JP-A 2002-167632) discloses one example of conventional rolled copper foils for a flexible printed circuit having a following configuration.
Namely, the rolled copper foil for a flexible printed circuit contains oxygen (O) of 100 to 500 mass ppm, and contains at least one element selected from a group consisted of silver (Ag), gold (Au), palladium (Pd), platinum (Pt), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os) in the ranges so as to control T defined by the following formula to 100 to 400: T=[Ag]+0.6[Au]+0.6[Pd]+0.4[Pt]+0.4[Rh]+0.3[Ir]+0.3[Ru]+0.3[Os] (wherein, [M] is the mass ppm concentration of the element M), and in which the total content of sulfur (S), arsenic (As), antimony (Sb), bismuth (Bi), selenium (Se) and tellurium (Te) is 30 mass ppm or less. The rolled copper foil has a thickness of 5 to 50 μm. The intensity (I) in the 200 plane obtained by X-ray diffraction for the rolled face after annealing at 200° C. for 30 min to the intensity (I0) in the 200 plane obtained by X-ray diffraction for the pulverized copper, i.e., I/I0 is >20. The rolled copper foil has a semi-softening temperature of 120 to 150° C., and continuously maintains tensile strength of ≧300 N/mm2 at a room temperature.
According to the above structure, the rolled copper foil for a flexible printed circuit disclosed by JP-A 2002-167632 has a superior bending-fatigue life characteristic.
However, in the rolled copper foil for a flexible printed circuit disclosed by JP-A 2002-167632, when oxide is generated from the oxygen (O) contained in this copper foil, there is the case that this oxide may become an origin of fatigue breakdown. Therefore, there is a limit for enhancement of the bending-fatigue life characteristic according to this structure.
Further, in the case of using the oxygen free copper containing substantially no oxide, since a softening temperature (apparent initial softening) of the oxygen free copper per se is higher than that of copper containing oxygen (O) (of 100 to 500 mass ppm), progression of recrystallization in the copper foil is insufficient under low temperature condition (e.g. 160° C.). Therefore, a good bending-fatigue life characteristic cannot be provided. Further, if additive elements disclosed by JP-A 2002-167632 is used in the copper, the softening temperature of the copper will be further raised. Therefore, the addition of the additive elements is advantageous under a high temperature condition (e.g. 400° C.). However, the copper containing such additive elements cannot be used under the low temperature condition. Still further, if no additive element is added to the oxygen free copper, there will be no affect of the oxide. Therefore, the progression of recrystallization advances in the copper foil appropriately under the low temperature condition, so that a good bending-fatigue life characteristic is provided. On the other hand, there is the case that the bending-fatigue life characteristic falls because the recrystallization advances excessively in the copper foil under the high temperature condition. Therefore, it is not possible for the conventional rolled copper to correspond to heat treatment in a wide temperature range.